VATICAN CITY – Pope Francis has ushered in his first Christmas as pontiff, likening Jesus' birth to a burst of light on the sometimes dark moments of human history, marked by pride and ambition.

The 77-year-old Francis celebrated Christmas Eve Mass late Tuesday in St. Peter's Basilica, returning to rest in his Vatican lodgings barely 12 hours before he was due to return to the basilica.

At noon (1100 GMT) Wednesday, Francis was scheduled to deliver a traditional Christmas day message meant for a worldwide audience. Tens of thousands of tourists and Romans are expected to gather in St. Peter's Square for his speech and blessing delivered from the central balcony of the basilica.

At the Tuesday night Christmas service, Francis drew on the humility theme of his nine-month-old papacy as he cited Jesus' humble beginning as a poor and vulnerable baby. "You are immense, and you made yourself small; you are rich, and you made yourself poor; you are all-powerful and you made yourself vulnerable," Francis said of Jesus as he delivered his homily in the basilica, packed with the faithful.

He noted that the first to receive news of Jesus' birth were shepherds, who in society were considered "among the last, the outcast."

The Argentine-born pope, who has also encouraged his flock to be a joyful church, called Jesus "the light who brightens the darkness."

In the world's history and our own personal history, Francis said, "there are both bright and dark moments, lights and shadows." He added "if our heart is closed, if we are dominated by pride, deceit, self-seeking, then darkness falls within us and around us."

Francis has applied this same vision to the heart of the Vatican's own working, saying in past remarks there is no place for personal ambition in the clerical hierarchy. Rather, he has insisted, the Catholic church must be one of service to those in need.

He quoted the Apostle John, saying "whoever hates his brother is in the darkness'" and "'does not know the way to go, because the darkness has blinded his eyes.'"

Earlier on Tuesday, in Bethlehem, the biblical birthplace of Jesus, the top Roman Catholic cleric in the Holy Land, Latin Patriarch Fouard Twal, also promoted the cause of brotherhood.

As thousands of Christian pilgrims from around the world packed the West Bank town, the patriarch described the message of Christmas as a "a message of peace, love and brotherhood. We have to be brothers with each other," he said.

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AP writer Mohammed Daraghmeh contributed to this report from Bethlehem.